Field devices, such as radar level gauges, are suitably used for measuring the level of products such as process fluids, granular compounds and other materials. An example of such a radar level gauge can include a microwave unit for transmitting microwaves towards the surface and receiving microwaves reflected by the surface, processing circuitry arranged to communicate with the microwave unit and to determine the level based on a relation between transmitted and received microwaves, an interface for connecting said processing circuitry externally of the radar level gauge, and a power management circuitry providing the microwave unit and the processing circuitry with operating power.
Radar level gauges for measuring a level in a tank, and other types of field devices, may often communicate with a remote location, such as a control room, by means of a current loop, such as a 4-20 mA industrial loop with or without superimposed digital communication.
For a 4-20 mA industrial loop interface without superimposed digital communication, the information which can be transmitted is limited in that only the magnitude of the current flowing through the current loop may be used to carry information. An example of a 4-20 mA industrial loop communication protocol capable of using superimposed digital communication is the HART-protocol.
In circumstances where the quality of the measurement value from the field device may be degraded, it is desirable to provide an indication to the host system that the measurement value has a reduced or degraded reliability. Accordingly, in a field device connected by a 4-20 mA industrial loop interface, an indication that the measurement value has a reduced reliability is commonly transmitted as a predetermined analog current value regardless of the reason and severity of the degradation of the reliability.
However, if the field device is part of a process control system, an indication from the field device that the measurement value has a reduced reliability may lead to a direct and unplanned interruption of the process. This means that a process may be interrupted by a less severe condition such as a reduction in accuracy. As such unplanned interruptions may be both costly and time consuming, it is highly desirable to reduce the occurrence of unplanned process events.